1. Field of the Invention
The present invention is related to batteries and a method for making batteries, and more specifically, the present invention is related to Lithium-Air batteries and a method for making Lithium-Air batteries.
2. Background of the Invention
The advancement and acceptance of electric cars has been hampered due to the lack of suitable batteries. Indeed, the 46 MJ/kg specific energy of gasoline makes replacement of this energy-dense fuel a daunting task. The theoretical specific energy of Li-air batteries is about 18.7 MJ/kg including oxygen. However, since oxygen is constantly drawn from the air for this battery, theoretical specific energy is increased to about 40.1 MJ/kg if the battery's oxidizer (oxygen) is not factored into the weight. This very closely approximates the specific energy of gasoline.
Due to engine inefficiencies, both gasoline and Li-air batteries are predicted to achieve a practical specific energy of 1,700 Wh/kg, which is several folds higher than most existing battery systems. This feature makes Li-air batteries ideal for long-range electric vehicles.
Li—O2 batteries are based on the use of an electrochemical transformation wherein formation of Li—O bonds occurs during battery discharge and the breaking of those bonds occur during charging of the battery. A porous oxygen gas-permeable cathode is used to store the solid products generated from the reaction of Li cations with oxygen gas. The general reactions of the redox sequence are presented as Equations 1-5, below:Anode Reaction Li(s)→Li++e−  Equation 1Cathode Reaction 2Li++2e−+O2Li2O2  Equation 2Cathode Reaction 2Li++2e−+0.5O2→Li2O  Equation 3Cathode Reaction Li+e−+O2LiO2  Equation 4Cathode Reaction 2LiO2Li2O2+O2  Equation 5
During discharge, lithium metal from the anode is oxidized to lithium cation during reduction of oxygen gas at the cathode to oxygen ion. The reverse redox occurs during recharging of the battery.
Despite the aforementioned promising specific energy values, state of the art Li-air batteries lag in several performance parameters. For example, their reported specific energy is 362 Wh/kg. While this is about 100 percent higher than Li-ion batteries, Li-air batteries only achieve 21 percent of expected practical value. Also, the specific power of Li-air batteries is about 10 percent of present Li-ion batteries.
Also, while power density is an essential parameter in electric propulsion, Li-air batteries are low in power density. During discharge, oxygen is reduced to form lithium-oxides. The charging cycle reverses this chemical reaction and produces oxygen gas. Both processes take place at the cathode surface. So to ensure satisfactory power output, a high surface area cathode is preferred. Li-air batteries fall short in round-trip efficiency, which is the ratio of energy discharged to the energy needed during charging. While 90 percent round trip efficiency is preferred for electric propulsion, Li-Air batteries with pure carbon cathodes display much lower efficiencies.
Life cycle is another limitation as Li-air batteries degrade after a very limited number of cycles. Per the equations above, lithium oxides form during discharging cycle as lithium ions are transferred to the cathode to react with incoming oxygen. The recharging process involves the reduction of lithium oxides (Li2O and Li2O2). However, Li2O is difficult to charge due to the broken O2 bond, as noted in Equation 3 above. Instead, it accumulates in the pore volume of the cathode, resulting in ultimate failure of the cathode. Separately, instability of electrolyte is a major cause of poor cycle life.
A need exists in the art for a lithium-air battery which has round trip efficiencies above approximately 90 percent. The battery should also have extended lifetimes of at least about 1000 discharge/charge cycles. Also, the battery should utilize only minimal amounts of expensive catalytic material (e.g., gold, silver, platinum, palladium) so as to make it accessible to a wider swath of car buyers.